DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Ammonite fossil. Thin section through a fossil of the ammonite Agathiceras timorensis. Ammonites are extinct cephalopod molluscs that lived in the oceans during the Jurassic and Cretaceous periods. A. timorensis lived 150 million years ago. A fossil is created when an animal is quickly buried in sand, mud or volcanic ash after death, and the normal decay processes do not occur. The minerals of the hard parts of the animal are gradually replaced with rock. The detailed septa (partitions) between the chambers of the shell's interior are particularly well seen. This specimen measures 2.5 centimetres in diameter. A. timorensis fossils are found in Indonesia.
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